In order to solder electronic parts on a printed board by melting a soldering material, a heating oven, such as a reflow soldering apparatus, is used. The reflow soldering apparatus is an apparatus in which a preheating zone, a heating zone, and a cooling zone are provided in a tunnel-like muffle, heaters are respectively provided in the preheating zone and the heating zone, and a cooling device composed of a water-cooling pipe or a cooling fan is provided in the cooling zone. For example, a printed board, to a soldering portion of which a soldering paste is applied by printing or the like, is transported to the respective zones and by melting the soldering paste of the printed board, electronic parts are soldered to the printed board.
In the heater used for the reflow soldering apparatuses, there are an infrared heater and a hot blast heater. The infrared heater emits infrared ray when the infrared heater is electrically conducted. The soldering paste applied to the soldering portion is melted by the emitted infrared ray to perform the soldering. It, however, has a problem such that it is difficult for the infrared heater to fully heat the soldering portion in the shadow of the electronic parts because the infrared ray travels in a straight line.
On the other hand, the hot blast heater has an advantage in that since a hot blast heated by the heater is circulated by convection by means of a fan rotated by driving of a motor in the heating zone of the reflow soldering apparatus, the hot blast penetrates in any parts in the shadow of the electronic parts and narrow clearances, thereby allowing the whole printed board to be uniformly heated. Today, the hot blast heater is often used in many reflow soldering apparatuses.
As the hot blast heaters installed in the reflow soldering apparatus, there are a heater which blows hot blast through a nozzle having a large opening area, and a heater which blows hot blast through a large number of apertures. The former heater has the nozzle with a large opening area so that a flow rate of hot blast relatively slows down, resulting in a rather poor heating efficiency when the hot blast collides with the printed board. In contrast, the latter heater has the apertures so that the flow rate is more increased than that of the former heater, and the heater provided with many apertures can avoid any shortage of hot blast. Thus, the latter heater achieves a high heating efficiency. Therefore, the reflow soldering apparatuses often use the heater configured to blow hot blast through a large number of apertures. A description hereinafter given relates to the hot blast heater having a plurality of apertures unless stated otherwise.
In the reflow soldering apparatus, heating is performed on the printed board in the order of the preheating and heating. In the preheating, the printed board is slowly heated by hot blast at low temperatures so that the printed board slowly adjusts to heat and a solvent contained in the soldering paste is volatilized. Preheating in the reflow soldering apparatus is preferably performed at lower temperatures by a smaller quantity of hot blast than that of the heating zone.
The printed board is heated in the heating zone of the reflow soldering apparatus after the printed board has slowly been adjusted to heat during the preheating and the solvent contained in the soldering paste has been volatilized so that the electronic parts have been relatively firmly bonded. In the heating, the soldering is performed by blowing hot blast at high temperatures so that a powdery solder material in the soldering paste is melted. A quantity of hot blast blasted to the printed board in the heating enables a temperature rise to accelerate if it is larger than a quantity of hot blast in the preheating. Heating is performed over a short period of time in the heating because a lengthy heat treatment under a high temperature possibly causes a thermal damage to the printed board and the electronic parts.
As a rule, the reflow soldering apparatus is equipped with a large number of hot blast heaters at respective positions above and below printed board transport units in the preheating zone for performing the preheating and the heating zone for performing the heating. For example, in a case where the preheating zone is composed of five zones, a total of ten hot blast heaters are equipped such that five heaters are respectively provided both above and below. In a case where the heating zone is composed of three zones, a total of six hot blast heaters are equipped such that three heaters are respectively provided both above and below. As a result, a total of 16 hot blast heaters are equipped in a reflow soldering apparatus with eight heaters being respectively provided above and below.
It is to be noted that such a zone configuration is suitably selected depending on types of electronic parts to be soldered to the printed board, in other words, the number of the heaters to be used or the like is suitably selected depending on a temperature profile of an object to be heated.
In the preheating zone and the heating zone, control of the flow rates and temperatures of the hot blast blown from the respective hot blast heaters by control means enables a desirable temperature profile suitable for the printed board to be set. Control of the hot blast temperatures by a temperature adjuster and variation of an output of a fan motor (hereinafter, referred to as “fan motor output”), which is attached to a fan, for rotating the fan cause to be controlled the flow rates of the hot blast blown into the muffle. In this connection, as such a motor, an inverter motor is generally used which is easy to control an output of the fan motor.
For example, the Patent Document 1 discloses an example of such a reflow soldering apparatus equipped with the heaters configured to blow hot blast through a large number of apertures. This heating oven is provided with a plurality of blast ports which blows hot blast and a plurality of recovery ports for forcibly recovering the hot blast blown through the plurality of blast ports and redirected after hitting an object to be heated. According to the heating oven, the hot blast cooled down by hitting the object to be heated and being redirected thereby is efficiently removed without the hot blast staying on a surface of the object to be heated so that heat exchanger effectiveness (heat transfer rate) on the surface of the object to be heated is improved, thereby uniformly heating the object to be heated.
On the other hand, the Non-Patent Document 1 discloses an impingement heat transfer of a cruciform jet, which is a non-circular shape. This impingement heat transfer of a cruciform jet relates to analysis of a jet blown through a cruciform blast port, based on an isothermal transmissibility distribution and an infrared image isothermal chart. An analysis result thereby obtained verifies the occurrence of a switching phenomenon in which the cruciform jet changes its shape with time such that projecting parts of the cross shape are flattened and dented parts thereof are projected.
Prior Art Documents
Patent Document
Patent Document 1: Japanese Patent Application Publication No. 2002-331357
Non-Patent Document
Non-Patent Document 1: Impingement Heat Characteristics of Cruciform Jet, Pages 233-239, No. 607 (March, 1997), Vol. 63 Collection of Literatures (Edition B), Japan Society of Mechanical Engineers